Electrolyte for and method of anodically polishing copper



- feasible temperatures.

s ms Jan. '9.

.aascraorm ron Ann moo ANODICALLY rousnmo coma Charles L. Fault. Columbus, Ohio, assignor to Battelle Memorial Illlflh lk, Columbus, 01110, a corporation of Ohio Nonim. Application April 24. 1942, Serial No. man

5 Claims. ('01; 204-140) This invention relates to a phosphoric acid contaming electrolyte for anodically polishing copper. The invention further pertains to methodsof electropolishing copper in such an electrolyte.

A bath heretofore preferred for the anodic polishlng of copper consists of 75% to 85% m phosphoric acid, saturated with chromic acid.

Depending on the temperature at .which the chromic acid is dissolved in the phosphoric acid, the composition oi. this bath will range from 69% orthophosphoric acid and 7% chromic acid to 81% orthophosphoric,.-acid and chromic acid.

'Minimuminitial current densities of 250 to 300 or even more amperes per square foot at tank voltages of 8 to 10 volts are required, and these minima are increased progressively as the bath is used. The viscosity of the phosphoric acidchromic acid bath is also somewhat high, causing excessive drag out loss oi electrolyte.

Orthophosphoric acid can be used alone for electro-polishing copper, but only at the impractically low temperature 01! 70 F. The addition oi hexavalent chromium in the form of chromic acid thus serves to make possible electropolish: ing oi. copper in phosphoric acid at practically But the progressive rise in current density andtank voltage characteristic of chromic acid containing phosphoric acid baths is thought to be due to this chromic acid addition, being caused, inore particularly, by continuous reduction of hexavalent chromium to trivalent chromium and by continuous accumulation in the bath of anodically dissolved copper.

I have now found that the addition oitrivalent aluminum and, optionally, trivalent chromium, to phosphoric acid containing copper polishing baths renders the baths less viscous and makes possibleelectropolishing at lower current densities, tank voltages and temperatures. Further, the dissolved copper is plated out cathodically in recoverable form from such baths, so that continuous operation is greatly facilitated and maintenance can be effected simply by replacing dragged-out electrolyte and decomposed water,

It is, therefore, an important object of the present invention to provide an electrolyte for the anodic polishing of copper containing phosphoric acid and trivalent aluminum together with trivalent chromium as an optional ingredient.

Another-important object of this invention is to provide methods for anodically polishing copper in phosphoric acid at low current densities and tank voltages operative at commercially desirable temperatures.

A further important object or the invention is to provide methods for continuously electropolishing copper in an electrolyte from which anodically dissolved copper is plated out cathodically in easily recoverable form.

.Still another object of the invention is to provide an electrolyte for the anodic polishing of copper characterized by substantially constant current density and tank voltage in continuous use and maintainable by simple additions to replace decomposed water and dragged out electrolyt Other andmirther objects and features oi. the present invention will become apparent from the following description and the appended claims. In general, the baths of the present invention contain preferably about 75 to 84% (by weight) 0! 100% orthophosphoric acid, about from 0.25 to 2.0% or, preferably, 0.3 to 1.5% trivalent aluminum, from 0 to 2% trivalent chromium, and,

after having been used, about from 0.3 to 0.5 up

to 1% copper, the balance being largely water. Such baths produce, with little or no agitation, a brilliant, mirror-like finish on copper at temperatures of from to or F. at current densities of from 50 to 200 or, preierably,

100 to amperes per square foot and tank voltages sometimes as low as 4 or 5 volts. The usual copper content oi. such baths is about 0.4 to 0.5%. Additional copper dissolved anodically is plated out on the cathodes where the copper may be recovered as a flne powder consisting of 98% or purer copper. Hence, the baths can be used continuously, with very simple chemical control and maintenance. The only replacements required .are those compensating for loss of decomposed water and for a limited amount of loss due to dragging out of the not very viscous electrolyte. The new baths are further less cor- Bath No. 1 Bath No 2 Per cent by Per cent by o: I o.s as

Balance Bath No. 1 may be prepared by adding a trivalent chromium salt to 75% orthophosphoric acid, the required amount of aluminum metal having been dissolved in the phosphoric acid by heating. For instance, chromic chloride may be dissolved in the phosphoric acid, the resulting solution being heated until all the hydrochloric 4 acid has been driven 01!, as evidenced by the 'acid, 1% chromic acid and 0.5% metallic aluminum dissolved in the phosphoric acid, this bath being electrolyzed and water being added to the bath until all the hexavalent chromium has been reduced to trivalent chromium.

Bath No. 2 may be prepared similarly to bath No. 1.

Copper is electropolished in bath No. 1, with very little or no agitation, to a brilliant, mirrorlike finish at 100 to 2 amperes per square foot at a temperature'o't' from 110 to 125 F. Bath No. 2 electropolishes copper well at a current density of 150 amperes per-square foot at a temperature of 120 F. The operative temperature ranges at various current densities for bath No. 2

after aging are set forth in-the following table:

Temperatures lower than the indicated minima may be employed but then provisions must be made for cooling the baths. At current densities of at least 200 amperes per square foot, temperatures of 170 F. or higher may be employed with ood results.

Although in both bath No. 1 and in bath No. 2 the electropolishingtime depends on the nature 01E the copper and the starting surface, it has been noted that bath No. 2 requires a slightly longer time than bath No. l, which has greater anode efllciency.

The relative eflect of temperature on cell voltage and current efllciency at various current densities inbath No. 2 after aging is shown by Table II.

Table II assume of divalent copper, trivalent chromium and trivalent aluminum) of less than 2.5%. Theinfluence of dilution on cell voltage and current emciency when electropolishing at1120 F. and a current density of 150 amperes per square toot is indicated in the following table:

The lower operative current densities at 120 F. are indicated in Table IV, which also serves to show the necessity for including trivalent aluminum with the bath to secure satisfactory electropolishing.

Table IV 76% HIPOl, 0.5% Al 84% Earl), 0.6% Al Current density cell Anode Anode amps w currcn cm 0 yo wrrent per sq. it efllc. in oiilc. $8 in permush 51:! in perpolish cent cent 3.5 39 Defect Good.-- 4.1 34 Good. ..do... 4.0 33 Do. 42 ...d0-.... 6.1 33 Do. 40 do. 6.6 33 Do. 40 -.do a: as Do.

85% HaPOI 3.3 44 Delec. 3.8 36 Do. 4.3 32 Do. 4.8 29 Do. 6.3 27 Do. 8.8 26 Do.

Table V further indicates the critical importance of the aluminum content of the baths according tothe present invention. This table. further shows that the presence and concentration of chromium are not critical. This table is in- Current den- If the water concentration is over 23 to 25% in the type of bath exemplified by baths Nos. 1 and 2, good electropolishing is not secured at a current density below amperes per square foot, at temperatures above 110 F., or at a metal 7 concentration (including the total concentration The eilect at 120 F. and a current density r 150 amperes per square foot of aging on baths according to the present invention and on baths T6016 7 Tabla VIII Q mmaluminum Noaluminnm mp Jrr flw ammo. am Or.0.4%Al 0.2%.01

mm m a volt m ssfgf' i m undo I involta Wt polish. luv to m M cathode 3i cathode W llt cw c n 9 (i 6.6 35 Good. 6.1 28 Dulllatin. voltage Chanoi voltage Ohanoi' vgg go Charm! 2 is g 2 fig go. h in polish in polish as as n 5.: D21 mu it h H 3% nila 1) pm 0 I a 1 a8 a 5.5 as Do. as will 2:: z: 2: t3 Y 1.3 n The necessity for a minimum aluminum con- 7 tent is still further shown in Table VI which (11- No. 2 i rectly shows the influence oi variousaluminum 0 concentrations on the nature of the electropolish as well as on the cell voltage and theicurrent Table Ix emciency at 120 F. and a current density of 20 W 0 i P 10 P0 150 amperes per sqxaze 13:. yr; 8 H1334 ag'r ngol,

. 0.4 -41". a la 0.0 ou 0.5 011" 0, ,c

17% HIPO 85% 1mm was. a.

Alconins e w vo l e k? vofi a ce m" contratiou can Anode cw Anode no W01 p0 lnpcroont voltage current Chanoi you current Char. oi 7 Mom eng i z i p i mm" we in P011111 3.5 Defoe-.- a1 Defoe--- a2 Defoe. p g as 0 a 4.3 Good. 7 0 5.4 D0. 0... 4.4 as DeiecL- 5.5 as Defect 6 agg 4 0 a: Gd has 32 0. 8175 4.; as 11102:: 0 .1 D3: Bath 1 .Bath 1.00 no as 0.2 as Do. 1.25 5.05 38 ---do.- 6.35 33 Do. M 33 35 Tables I to 1x include data showing that the inclusion oi. trivalent aluminum with phosphoric acid makes possible continuous electropolishing of copper in the resulting bath, dissolved copper being recovered as electropolishing is continued.

containing phosphoric acid saturated with 40 The presence of limited amounts of trivalent chromic acid is shown by Table VII, which also indicates the influence oi cathode size. The progressive deterioration due tohigh chromic acid content is evident.

chromium is not harmful, but does not add to the efliciencyot the bath. A larger cathode area lowers cell voltage and raises current eihciency for plating out copper during electropolishing.

Table VII Original mm composition 847 m 0. Original we om sition 1% crol, 0.5% A1 9 211% HIPO, a {2:0

- h Current efll- Cell Cur. Cell voltage Per cent in volts 231W a in cop or Chat. Char. disso vsd oi pol. o! pol. at mode I! 2%" KI! 2%" II II I cathode cathode cathode cathode eat a caghods 0 5.1 4a a4 34 Good. so 42 Good. 2 5. 1 a a a4 34 ...do... a a 41 Do. 4 a 1 4. 4 a4 34 d n a 0 as Do.

o a c 4 4 a4 34 r n as 30 Del. i a a4 4.4 34 34 r 0 a4 30 Do 10 a a a 4 a4 34 --.c o..- a 4 to Do. 12 5. 3 4. 4 34 34 ..c 0.-- 6. 4 307 Do. 14 a a a 4 a4 34 do a 4 a0 Do.

The bath having an original composition in-- cluding 84% HaPO4, 1% CrOa and 0.5% A1 is similar to bath No. 2 discussed on page-4. The

amounts of copper indicated as dissolved do not necessarily indicate a corresponding copper content in the baths, since some of the dissolved copper will be plated out. In the case of the alumiof cathode size and of chromium content on 888d baths at 120 F.

As indicated, many changes in bath composition and in polishing procedure can be eflectedthrough a wide range without departing from the principles of this invention, and it is therefore not my purpose to limit the patent-granted on this invention otherwise than necessitated by the scope of the appended claims.

What I claim is:

1. An electrolyte ioruse in the anodic polishing of'copper containing from to 84% by weight ortho phosphoric acid and from 0.25 to 2% trivalent aluminum, the balance comprising essentially water.

I 2. An electrolyte for use in the anodic polishing of copper containing from '15 to 84% by weight ortho phosphoric acid, from 0.25 to 2% trivalent aluminum, and from 0.3 to 1% divalent copper, the balance comprisir'rg essentially water.

3. The method of electrolytically polishing copper which comprises making the copper the anode in an electrolyte comprising from 75 to 84% by weight ortho phosphoric acid and from 0.25 to 2% trivalent aluminum, the balance comprising essentially water, and at a temperature of at least 100 F. passing current therethrough of sufllcient density and for a sufflcient period of time to effect a polish on said copper.

4. The method of electrolytically polishing copper which comprises making the copper the anode in an electrolyte containing from '75 to 84% by weight .ortho phosphoric acid, from 0.25 to 2% trivalent aluminum, and from 0.3 to 1% divalent copper, the balance comprising essentially water, and at a temperature of at least 100 F. passing current through said electrolyte at a sufflcient current density and for a suiiicient period of time to effect a polish on said copper.

5. The method of electrolytically polishing copper which comprises making the copper the anode by weight orthophosphoric acid, from 0.25% to 2% trivalent aluminum, and from 0.3 to 1% divalent copper, the balance comprising essentially water, and passing a current through said elec trolyte at a temperature of from 100 to 170 F. at a current density of from to 250 amperes per square foot, the exact current density depending upon the temperature as indicated by the follow ng table:

Current density and amperes per Minimum Maximum square fOOt temperature temperature said current being passed through said e1ectrolyte for a sufiicient period of time to effect a polish on said copper.

CHARLES L. FAUST. 

